1. Field of the Invention
The present invention relates to a CMOS output circuit, and more particularly to a dynamic CMOS output circuit with transfer gates.
2. Description of the Prior Art
A CMOS output circuit for providing a multi-level drive signal based on a control signal is formed in general on a silicon substrate and is profitably employed as a display drive circuit for driving a liquid crystal display (LCD) panel.
Recently, liquid crystal display (LCD) panels have been widely employed as a low power consumption display device for electronic equipment. The LCD display panel includes a plurality of display elements, each of which has a liquid crystal layer sandwiched between two electrodes which responds to a drive voltage applied between the two electrodes to vary its molecular orientation. The molecular orientation controls the scattering or transmission of light impinging upon the liquid crystal layer. LCD panels are generally operated by AC drive signals to prevent the degradation of its liquid crystal material.
For example, in the operation of a LCD panel which includes a plurality of X and Y electrodes in a matrix form and liquid crystal layers between the X and Y electrodes, a CMOS output circuit has been used for providing drive signals of multi-levels (for example, +5 V, +3 V, +1 V, -1 V) to the X and Y electrodes of the LCD panel by selectively controlling different reference voltages.
However, a prior art CMOS output circuit has the following disadvantages.
If the voltage difference between the maximum reference voltage and a minimum reference voltage of a plurality of reference voltages becomes greater than a predetermined value, (for example, more than 14 V), the CMOS inverters and transfer gates in the CMOS output circuit may be latched up, so that an uncontrolled large current flows through the CMOS inverters and transfer gates. It would be impossible to stop the uncontrolled current in such a condition without removing an external power supply voltage.
In particular, since a large sized LCD panel requires a high voltage signal as a driving signal to obviate the degradation of the screen contrast, it is very important to solve the latching problem.
Also, if a voltage difference between the maximum and minimum voltages of a plurality of reference voltages becomes extremely low (for example, less than a threshold voltage V.sub.T), the CMOS output circuit cannot provide enough drive current to the LCD panel. In order to improve the current supplying capability of the CMOS output circuit, the use of a large MOS transistor will result in the increase of the IC chip area.
Meanwhile, there has been another strong desire to provide a CMOS output circuit which can operate with a lower power supply voltage (for example, about 5 volts) without decreasing its current supplying capability to the LCD panel.
In order to overcome the two disadvantages noted above, there is a need for a CMOS output circuit which can operate over a wide range of power supply voltages.